Abstract: A method for producing a vapor stream from waste plastic comprises providing a waste plastic feedstock into a reactor containing one or more residues produced from a previously heated source of waste plastic, and heating the waste plastic feedstock in the reactor to a temperature from about 125° C. to 500° C. to generate a vapor containing one or more hydrocarbons. The waste plastic feedstock can have a calcium to sodium mass ratio from about 0.0001 to 400 as measured by inductively-coupled plasma (ICP) spectrometry. The catalytic activity in the reactor may be provided through one or more constituent elements in the waste plastic feedstock or the one or more residues produced from the previously heated source of waste plastic.